This invention relates to methods of casting and is particularly directed to a method of sand casting metal parts having smooth, flat surfaces to be held within close dimensional tolerances, e.g., tolerances of the order of 0.010".
The present invention is particularly useful in casting large parts, for example, valve bodies, which may weigh as much as several hundred or even several thousand pounds. Many such valve bodies include various smooth surfaces which are intended to mate with other parts of the valve. These surfaces must be smooth and frequently must be held within close tolerances, for example, tolerances of the order of 0.010" .
In the past it has been customary to cast articles, such as the valve bodies described, without any attempt being made to cast the smooth, precisely-located surfaces. Rather, it was customary for the foundry to leave large excesses of metal in the areas where the surfaces were to be located so that the surfaces could be machined in a conventional way by turning, milling or the like. In addition to the inclusion of various machined surfaces, many past cast parts have other dimensional requirements. Thus, many such parts are intended for use under high pressure or other conditions in which the parts are subjected to substantial stresses so that certain wall thicknesses and the like must be above a critical dimension.
In conventional foundry practice, no effort has been made to closely regulate the thickness of cast walls. Rather, as a practical matter, foundries made the wall thicknesses substantially thicker than necessary in order to make certain that even under adverse conditions the walls exceeded the required minimum thickness.
As a result of the lack of a method for precision sand casting large parts, the cost of castings were substantially increased by the inclusion of excess material both in areas where it was used to insure minimum wall thickness and in other areas where it was initially provided and subsequently machined away. Additionally, the cost of many parts was substantially increased by the intricate machining operations necessary to finish and precisely locate various surfaces on the part.